We have identified a potent tolerogenic plasmacytoid dendritic cell (pDC) population defined by expression of the chemokine receptor CCR9, and have shown that these CCR9+ pDCs not only induce peripheral tolerance by induction of regulatory T cells (Tregs), but also efficiently transport innocuous peripheral antigens (Ags) to the thymus where they mediate central tolerance through clonal deletion. In dendritic cell (DC) immunotherapeutic approaches we also confirmed an immunosuppressive role for CD103-expressing conventional DCs (cDCs) that also have the ability to access the thymus and delete Ag-specific thymocytes. We therefore hypothesize that specialized DC populations play related or complementary roles in central tolerance to peripheral Ags. Moreover, we propose that CD103 is important in this process by either affecting DC microenvironmental localization within the thymus or their interaction with developing thymocytes. We also suggest that thymic-homing peripheral DCs mediate central tolerance to orally acquired (dietary) Ags; and that they are critical to normal immune homeostasis and prevention of autoimmune pathologies. Studies under Aim 1 will therefore define the role of peripheral cDC subsets in thymic transport of peripheral Ags and in the induction of central tolerance. The mechanisms of tolerogenic DC trafficking to and microenvironmental localization and interactions within the thymus will be studied using bone marrow radiation chimeras, competitive homing studies and parabiosis experiments. Studies under Aim 2 will establish the role of peripheral DCs, their tissue source and trafficking properties in mediating central tolerance to fed Ags. These studies will involve inducible transgenic approaches, to temporarily delete DC populations, followed by DC add back experiments during Ag feeding. Finally, studies under Aim 3 will evaluate the importance of thymic homing DCs and the trafficking and adhesion receptors, CCR9 and CD103, in the maintenance of immune homeostasis, including suppression of MOG-driven EAE pathology.